Further to yesterday's post, (which sprang from a thread in the DIY group on Flickr), a kind person called Stan160 gave me the link to Silverprint which is just about bloody brilliant. You get half a kilo of Sodium sulphite (anhydrous), for example, for £3.10, which, my brief glances at the digital truth photo recipes suggests is enough, with much smaller amounts of other chemicals, to make up 5 litres of developer. Splendid. And much more fun than hunting around Glasgow for a bottle of bloody Retinol, which is what I've been doing.
So, I've got a litre of Perceptol to mix up, and when that's done, I'm going to go DIY all the way.